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Multifunctional graphene/silicone nacre-like composite films

Periodic Reporting for period 1 - SILGRAFUN (Multifunctional graphene/silicone nacre-like composite films)

Reporting period: 2016-05-01 to 2018-04-30

Lightweight, conductive and mechanically robust composites are key for the development of a variety of hi-tech industries of high socio-economic relevance. When processing such composites, nanoscale events govern the bulk properties and some continue to be not fully understood. This has delayed the industrial uptake of nanocarbons in the composite industry and the fulfilment of the scientific expectation of conferring extraordinary properties to polymers at low filler content.
In the case of graphene, it has been found that this transference of properties is mostly limited to the number of layers found in the original graphenic derivative. Unfortunately, predominantly monolayer graphene in high quality and large quantity is still an infrequent product. Additionally, due to its inertness, pristine graphene results incompatible with most solvents and polymers. This entails functionalisations or modifications that are not easily pushed to completion prior to compositing and can hamper the electrical conductivity of the pristine sp2 carbon lattice. Graphene oxide (GO) in contrast, although less conductive when reduced and having limited solubility in organic solvents, is mass-producible. Its oxygen-containing functional groups enable access to high aspect ratio water-based suspensions with monolayer contents above 95%, both crucial characteristics to account for high quality composite production.
In this context, SILGRAFUN´s objectives have been to develop a novel strategy for processing both graphene and GO composite films and address their study as multifunctional materials for applications such as anticorrosive coatings, strain sensors or high-K materials. Using polymers containing pyrene- and NH2-dangling units, we have aimed to stabilise graphene and GO, respectively, in organic solvents and polydimethylsiloxane (PDMS)-based polymers. We propose a system in which the polymeric functionalisation that is created on the surface of GO can be opportunely removed: polymer brushes that enable dispersing GO in an organic PDMS matrix can be cleaved through UV-light once their dispersing function has been exploited. Since bulk conductivity and mechanical reinforcement change from the original functionalised state to the photo-cleaved one, in this study we provide an insight into how the nature of the graphene or rGO-interface alters morphologies, and in turn the inter-particle distance, of graphene or rGO/PDMS composites. We believe our study could have a great impact on a new generation of smart graphene-based composites with highly targeted final properties.
In the first half of the project we dedicated our efforts towards the synthesis and characterisation of the polymers to be used as stabilisers of graphene. We had to vary the synthesis path several times until a sufficiently high yield reaction could be found. Once the pyrene-containing polymers were ready and their characterisation complete, we aimed at the stabilisation of home-exfoliated graphene nanoplatelets (GNPs). We realised that the extent of stabilisation was highly dependent on the aspect ratio (hence number of layers) of the initial material. To fully monitor and explain the destabilisation of these hybrids with UV-light we first needed a system where the GNPs could be completely stable (extended time) before triggering the destabilisation. We decided to redesign a route where we could ensure maximal initial stability of the graphene layers, thus we chose GO. This new approach forced us to change the anchoring unit of our PDMS brushes and we introduced -NH2 functionalities instead. With these GO/PDMS hybrids we studied the photo-cleavage process in PDMS-based formulations and solvents and we were able to relate the resulting interlayer distances (before and after shining UV light) with the bulk properties of the final composites. These were then tested as strain sensors, anticorrosive coatings and dielectric actuators (before reduction of GO).
MILESTONES:
-Synthesised and fully characterised novel cleavable and non-cleavable polymer brushes based on PDMS, a photo-cleavable derivative and either pyrene or -NH2 anchoring units to stabilise graphene nanoplatelets (GNPs) and GO flakes, respectively. -Fabricated photo-responsive rGO composites
-Studied the rheological and structural properties of the rGO functionalised platelets in relation to the resulting morphology of the graphene sheets (inter-sheet spacing in a liquid crystal order or randomly oriented)
-Produced of compliant rGO/PDMS films with high barrier properties that have been successfully tested as efficient anticorrosive coatings and strain sensors
-Contributed to the consistent understanding of how the structure of graphene/polymer composites on different scales affects the bulk properties and therefore on how to design smarter materials to meet application requirements

This research programme has enabled Dr. Crespo acquiring expertise in several fields (e.g. chemical synthesis, polymer chemistry, graphene science, nano self-assembly, advanced composite processing, electroanalytical characterization), hence her multidisciplinary profile has been strengthened. The publications associated to this piece of research will impact her career very positively. MCR has created links that will be very fruitful for her next steps, as there are currently other collaboration works in process.
SILGRAFUN has contributed to accelerate the “uptake of nanotechnologies, advanced materials or advanced manufacturing and processing technologies” inthe EU. This project has contributed to keep the EU to the forefront of graphene based technologies (patent to be filed) and benefited the EU´s industry competitiveness. Furthermore, we have currently 3 publications being prepared, one of which will be ready for submission within few weeks. Once our publications are released, we will ensure maximal diffusion to help explain the societal impact that the development of new multifunctional, lightweight and nanostructured composite materials for emerging technologies has. Dr Crespo, Dr. Gaurtrot and Dr. Bilotti will ensure that the outcomes of this project are further spread in upcoming International Conferences as they have included these results in their portfolio for inciting collaborative networking.
This fellowship has addressed the study and production of graphene/silicone layered materials for multifunctional applications. We have achieved tailor-made properties by controlling the composite´s microstructure throughout UV-triggered chemical reactions at the molecular level. As a proof-of-concept we have been able to couple this protocol with simple manufacturing methods to produce compliant composite films that have shown different performance depending on the extent of UV exposure. Our results have not only contributed to explain how graphene oxide behaves when dispersed in a stable fluid (as a liquid crystal, LC), but has also generated a consistent linkage between the controlled alteration of graphene´s percolation network and the resultant bulk properties of the composites. We believe our study could have a great impact on a new generation of smart graphene-based composites with highly targeted final properties. This study calls for a more systematic investigation design of molecular parameters regulating such responsive behaviour and their integration to processing methodologies for structural and energy management materials.
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